That's a great question. I just checked. We started the channel mid Dec 2015, and the first Adwords was week starting 7 Mar 2016. The first few weeks were basically experiments and learning (and we had some great advice and pointers from FamilyToyReview).
I think in March 16 we had 1265 subscribers.
When we started, Adwords was used to bring traffic up for a video so it could rank and show up in search. Without it, a video generally got 200-1000 views. This was not enough to rank. With $10-$20 that goes up to 2000-3000 views. Not much in the Yt world, but because of the way things work, that is enough to bypass thousand and thousands of other startup channels and their videos and get higher search placements. When this happens across a wide variety of videos (and keywords), the channel gets greater exposure overall, much more so than the 34,576 other kids channels all competing for viewers.
Whether to use Adwords or not goes to the heart of how you believe Yt works. I believe it is a highly tiered and layered system. The most critical thing to growth is growth of subs and views. If this does not grow, the channel will remain in the lowest tier. Once you level up to 1k, 2k, 5k subs, and 1k, 2k, 5k, 10k per video, the algorithm bumps you up to higher tiers. This is where the absolutely most vital aspect of getting traffic happens - getting into suggested slots of high tier channels.
I think the Adwords strategy we used has been instrumental in our channel growth. But it is expensive, and you need a long term view, and belief in your content and overall business. But what business isn't expensive? A guy just opened a pizza shop a few streets away, it cost him $170,000 and he's trying to flog $5.99 large pizzas (I'm eating a slice now).
